Polyethylene-2,6-naphthalene dicarboxylate resin and preform and bottle molded thereof

ABSTRACT

A polyethylene-2,6-naphthalene dicarboxylate resin for molding a bottle, which comprises 2,6-naphthalenedicarboxylic acid as a main acid component and ethylene glycol as a main glycol component and contains a germanium compound as a polycondensation catalyst, and has an intrinsic viscosity of 0.55 to 0.75 dl/g, a terminal carboxyl group content of 32 eq/ton or less, an acetaldehyde content of 8 ppm or less and a diethylene glycol component content of 0.8 to 3.0 wt %.

DETAILED DESCRIPTION OF THE INVENTION

This invention relates to a polyethylene-2,6-naphthalene dicarboxylateresin and to a preform for a bottle and a bottle, which are moldedthereof. More specifically, it relates to a polyethylene-2,6-naphthalenedicarboxylate resin which is excellent in hue, transparency, thermalstability and moldability, to a preform for a bottle and to a bottlemolded thereof which is excellent in hue, transparency and thermalstability.

Plastic bottles are mainly used as bottles for beverages such as juice,water, beer or the like. Materials for these plastic bottles need to beexcellent in hue, transparency, gas barrier properties and flavorretention properties from a viewpoint of increasing commercial productvalue when a bottle and its contents are seen as a unitary commodity.

For the purpose of saving resources and the like, recycling of plasticbottles, that is, the re-use of bottles has been recently studied. Arecycle system for the bottles comprises the step of cleaning a bottlewith an alkali at a high temperature. Therefore, the bottle must have aresistance to hydrolysis to stand this step.

Polyethylene-2,6-naphthalene dicarboxylate (may be abbreviated as PENhereinafter) can be basically produced using the same catalyst as thatfor polyethylene terephthalate (may be abbreviated as PET hereinafter),but from a viewpoint of hue, germanium dioxide is used as apolymerization catalyst for PEN, in particular.

According to studies conducted by the inventors of the presentinvention, whitening occurs more readily at the time of blow molding PENthan PET, and a bottle having satisfactory transparency cannot beobtained from PEN with the prior art.

The present inventors have further studied the cause of whitening of abottle and have found that whitening of a bottle is caused by depositedparticles derived from a catalyst contained in a polymer. That is, thedeposited particles derived from a catalyst induce crystallization andincrease internal haze. Particularly, in the case of PEN, stress at thetime of molding (stretching) a bottle is larger compared with PET andhence, the influence of crystallization largely results.

Therefore, to suppress the whitening of a bottle, the amount of adeposit derived from a catalyst or the like must be reduced.

There is proposed a method for reducing the deposit, in which a specificcatalyst is used in a specific amount and a specific ratio. It is forsure that the transparency of PEN is improved by this method.

However, even with this method, PEN having satisfactory hue and thermalstability could not be obtained. It has been found that the reason forthis is that the decomposition rate of a polymer becomes high when thepolymer is melt molded because the catalyst is not deactivatedcompletely due to a small amount of a phosphorus compound added which isused to deactivate the ester exchange catalyst so as to suppress theproduction of a deposit in the prior art.

It has been also revealed that when the melt viscosity of the polymer isnot appropriate at the time of melt molding PEN, such problems arisethat the resulting bottle cannot have a smooth surface and uniformthickness or a haze generates due to an extreme deterioration of itshaze value or a large amount of acetaldehyde is regenerated.

When the amount of acetaldehyde contained in the raw material polymer islarge in addition to the amount of regenerated acetaldehyde at the timeof molding, the amount of acetaldehyde contained in a preform and abottle is increased consequently, whereby an offensive odor derived fromacetaldehyde is generated from the contents of the bottle.

Since PEN has low impact strength, a PEN bottle is easily cracked ordelaminated when dropped.

JP-A 7-48440 (the term "JP-A" as used herein means an "unexaminedpublished Japanese patent application") discloses a process forproducing an aromatic polyester as a raw material for providing a filmhaving excellent surface flatness and resistance to dry heatdeterioration. In this method, a magnesium compound, calcium compound,germanium compound and phosphorus compound are added to a reactionsystem in amounts that satisfy the following expressions (11) to (14)before a polymerization reaction goes to completion:

    2.1≦(Mg+Ca)≦5.1                              (11)

    1.0≦Mg/Ca≦6.0                                (12)

    1.6≦(Mg+Ca)/P ≦7.0                           (13)

    0≦Ge≦1.5                                     (14)

wherein Mg, Ca, Ge and P indicate the numbers of moles of elementmagnesium, element calcium, element germanium and element phosphorus,based on 10⁶ g of an aromatic dicarboxylic acid component constitutingthe aromatic polyester, respectively.

JP-A 7-109340 discloses a polyethylene naphthalene dicarboxylatecomposition which contains magnesium, calcium, phosphorus and germaniumcompounds in amounts that satisfy the following expressions (21) to(24):

    2.6≦(Mg+Ca)≦4.1                              (21)

    1.3≦Mg/Ca≦6.0                                (22)

    1.0≦P/(Mg+Ca)≦1.5                            (23)

    1.0≦Ge≦3.0                                   (24)

wherein Mg, Ca, P and Ge are the same as defined in the aboveexpressions (11) to (14), and an ammonium compound represented by thefollowing formula (25) in an amount of 0.04 to 0.4 mole based on 10⁶ gof an acid component constituting the polyester:

    (R.sup.1 R.sup.2 R.sup.3 R.sup.4 N.sup.+)A.sup.31          (25)

wherein R¹, R², R³ and R⁴ are each a hydrogen atom, an alkyl group, acycloalkyl group, an aryl group or a derivative group thereof or R³ andR⁴ may form a ring, and A⁻ is a residual anion group.

JP-A 8-113632 discloses polyethylene naphthalene dicarboxylate for abottle, which contains magnesium, calcium, phosphorus and germaniumcompounds in amounts that satisfy the following expressions (31) to(34):

    2.6≦Mg+Ca≦6.0                                (31)

    1.3≦Mg/Ca≦6.0                                (32)

    1.0≦P/(Mg+Ca)≦1.5                            (33)

    1.0≦Ge≦3.0                                   (34)

wherein Mg, Ca, P and Ge are the same as defined in the aboveexpressions (11) to (14), and a phosphonium compound represented by thefollowing formula (35) in an amount of 0.03 to 0.5 mole, based on 10⁶ gof an acid component constituting the polyester:

    (R.sup.1 R.sup.2 R.sup.3 R.sup.4 P.sup.+)A-                (35)

wherein R¹, R², R³, R⁴ and A⁻ are the same as defined in the aboveformula (25).

JP-A 8-92362 discloses polyethylene naphthalene dicarboxylate for abottle, which contains magnesium, calcium, phosphorus, germanium andcobalt compounds in amounts that satisfy the above expressions (31),(32) and (34) and the following expressions (41) and (42):

    0.1≦Co≦0.4                                   (41)

    1.0≦P/Co+Mg+Ca≦1.5                           (42)

wherein P, Mg and Ca are the same as defined in the above expressions(11) to (13) and Co indicates the number of moles of element cobalt,based on 10⁶ g of a dicarboxylic acid component constituting thepolyester, and an ammonium compound represented by the above formula(25) in an amount of 0.04 to 0.4 mole, based on 10⁶ g of an acidcomponent constituting the polyester.

Further, JP-A 9-77859 discloses polyethylene naphthalene dicarboxylatefor a bottle, which contains cobalt, magnesium, calcium, phosphorus andgermanium compounds in amounts that satisfy the following expressions(51) to (55):

    0.05≦Co≦0.40                                 (51)

    2.0≦Mg+Ca≦6.0                                (52)

    1.3≦Mg/Ca≦6.0                                (53)

    1.0≦P/(Co+Mg+Ca)≦1.5                         (54)

    1.0≦Ge≦3.0                                   (55)

and further an ammonium compound having an intrinsic viscosity, measuredat 35° C. in a mixed solvent consisting of 3 parts by weight of phenoland 2 parts by weight of tetrachloroethane, in the range of 0.60 to 0.80dl/g and represented by the above formula (25) in an amount of 0.04 to0.4 mole, based on 10⁶ g of an acid component constituting the polyesteras required.

However, the polyesters disclosed by the above prior arts are not allsatisfactory as a raw material for a bottle.

It is therefore an object of the present invention to providepolyethylene-2,6-naphthalene dicarboxylate for molding a bottle which isof high grade and excellent in hue, transparency, thermal stability andmoldability.

It is another object of the present invention to provide a preform for abottle and a bottle which are formed from the abovepolyethylene-2,6-naphthalene dicarboxylate of the present invention.

It is a further object of the present invention to provide a bottlewhich can be recycled as it is and retains hue and transparency evenafter dozens of recycle uses.

Other objects and advantages of the present invention will becomeapparent from the following description.

According to the present invention, firstly, the above objects andadvantages of the present invention can be attained by apolyethylene-2,6-naphthalene dicarboxylate resin for molding a bottlewhich comprises 2,6-naphthalenedicarboxylic acid as a main acidcomponent and ethylene glycol as a main glycol component and contains agermanium compound as a polycondensation catalyst, has an intrinsicviscosity of 0.55 to 0.75 dl/g, a terminal carboxyl group content of 32eq/ton or less, an acetaldehyde content of 8 ppm or less and adiethylene glycol component content of 0.8 to 3.0 wt %.

The polyethylene-2,6-naphthalene dicarboxylate of the present inventionis a polyester comprising 2,6-naphthalenedicarboxylic acid as a mainacid component and ethylene glycol as a main glycol component.

The 2,6-naphthalenedicarboxylic acid as a main acid component ispreferably contained in an amount of 70 mol % or more, preferably 80 mol% or more, particularly preferably 85 mol % or more, based on the totalof all dicarboxylic acid components.

A subsidiary acid component other than the 2,6-naphthalenedicarboxylicacid can be preferably contained in an amount of less than 30 mol %,more preferably less than 20 mol %, particularly preferably less than 15mol %, of the total of all dicarboxylic acid components.

Specific examples of the subsidiary acid component other than2,6-naphthalenedicarboxylic acid include 2,7-naphthalenedicarboxylicacid, 1,5-naphthalenedicarboxylic acid, 1,7-naphthalenedicarboxylic acidand other isomers of naphthalenedicarboxylic acid; aromatic dicarboxylicacids such as terephthalic acid, isophthalic acid, diphenyldicarboxylicacid, diphenoxyethanedicarboxylic acid, diphenyletherdicarboxylic acidand diphenylsulfonedicarboxylic acid; alicyclic dicarboxylic acids suchas hexahydroterephthalic acid and hexahydroisophthalic acid; aliphaticdicarboxylic acids such as adipic acid, sebacic acid and azelaic acid;and the like. These dicarboxylic acid components may be used alone or incombination of two or more.

Ethylene glycol as a main glycol component is preferably contained in anamount of 70 mol % or more, more preferably 80 mol % or more,particularly preferably 85 mol % or more, based on the total of allglycol components.

A subsidiary glycol component other than ethylene glycol can bepreferably contained in an amount of less than 30 mol %, more preferablyless than 20 mol %, particularly preferably less than 15 mol %, of thetotal of all glycol components.

Specific examples of the subsidiary glycol component other than ethyleneglycol include trimethylene glycol, tetramethylene glycol, hexamethyleneglycol, decamethylene glycol, neopentyl glycol, diethylene glycol,1,1-cyclohexane dimethanol, 1,4-cyclohexane dimethanol;2,2-bis(4'-β-hydroxyphenyl)propane;bis(4'-β-hydroxyethoxyphenyl)sulfone, and the like. These glycolcomponents may be used alone or in combination of two or more.

An oxyacid such as p-β-hydroxyethoxybenzoic acid or ε-oxycaproic acidmay be used in place of all or part of the above subsidiary dicarboxylicacids.

As a copolymerizable component constituting a copolymer in the presentinvention, a polyfunctional compound can be further used in such anamount that the copolymer is substantially linear, for example, 2 mol %or less based on the total of the whole acid component. Specificexamples of the polyfunctional compound include polycarboxylic acids andpolyhydroxy compounds having 3 or more functional groups, such astrimellitic acid and pentaerythritol.

The polyethylene-2,6-naphthalene dicarboxylate of the present inventionincludes not only a homopolymer but also a copolymer, as describedabove.

The polyethylene-2,6-naphthalene dicarboxylate of the present inventionhas an intrinsic viscosity of 0.55 to 0.75 dl/g.

When the intrinsic viscosity is smaller than 0.55 dl/g, the polymer isliable to be crystallized and whitened in molding it into a preform or abottle. In addition, due to its too low melt viscosity, its blowmoldability is poor, thereby causing the decentering and nonuniformthickness of a bottle. On the other hand, when the intrinsic viscosityis larger than 0.75 dl/g, a molding failure occurs at the time when itis molded into a preform at a normal molding temperature, or the amountof acetaldehyde produced by heat generated by shearing increases,thereby causing a great reduction of intrinsic viscosity. The intrinsicviscosity is preferably 0.58 to 0.73 dl/g.

The polyethylene-2,6-naphthalene dicarboxylate of the present inventionmust contain terminal carboxyl groups in the polymer in an amount of 32eq/ton or less. When the amount is more than 32 eq/ton, hydrolysis isaccelerated during washing with an alkali, thereby reducing the strengthof the resulting bottle or degrading the outer appearance including adecrease in transparency.

The amount of terminal carboxyl groups is preferably 30 eq/ton or less,more preferably 2 to 30 eq/ton.

The polyethylene-2,6-naphthalene dicarboxylate of the present inventionmust have an acetaldehyde content in the polymer of 8 ppm or less. Whenthe content is more than 8 ppm, the total amount of acetaldehydegenerated by melt molding and acetaldehyde present before moldingbecomes excessive in the resulting preform or bottle, thereby giving anoffensive odor to its contents. The content of acetaldehyde ispreferably 6 ppm or less, more preferably 0 to 5 ppm.

The polyethylene-2,6-naphthalene dicarboxylate of the present inventionmust have a diethylene glycol content in the polymer of 0.8 to 3.0 wt %.If the content is less than 0.8 wt %, the transparency is deterioratedand the strength of the resulting bottle is reduced by the accelerationof crystallization. When the content is more than 3.0 wt %, thermalstability is lowered by thermal decomposition. The content of adiethylene glycol component is preferably 1.0 to 2.8 wt %, morepreferably 1.2 to 2.5 wt %.

The polyethylene-2,6-naphthalene dicarboxylate of the present inventionpreferably has a b value in the range of +0.6 to -6.0, more preferably 0to -5.0.

The polyethylene-2,6-naphthalene dicarboxylate of the present inventioncan be produced by melt polycondensation using a cobalt compound,magnesium compound and calcium compound as ester exchange catalysts anda germanium compound as a polycondensation catalyst. A phosphoruscompound is preferably used as a stabilizer.

Therefore, the polyethylene-2,6-naphthalene dicarboxylate of the presentinvention preferably contains a cobalt compound, magnesium compound,calcium compound and phosphorus compound in amounts that satisfy thefollowing expressions:

    0.12≦Co≦1.2                                  (1)

    2.0≦Mg+Ca≦6.0                                (2)

    1.3≦Mg/Ca≦6.0                                (3)

    1.0≦P/(Co+Mg+Ca)≦1.5                         (4)

    1.0≦Ge≦3.0                                   (5)

wherein Co, Mg, Ca, P and Ge indicate the number of moles of cobaltatoms in the cobalt compound, the number of moles of magnesium atoms inthe magnesium compound, the number of moles of calcium atoms in thecalcium compound, the number of moles of phosphorus atoms in thephosphorus compound and the number of moles of germanium atoms in thegermanium compound, based on 1 ton of acid components constituting thepolymer, respectively.

The cobalt compound functions as an ester exchange reaction catalyst andbesides, suppresses yellowing which causes degradation of the hue of thepolymer. When the amount of the cobalt compound added is less than 0.12mole, the effect on improvement of hue can be hardly expected, and whenthe amount is more than 1.2 moles, the hue of the polymer degrades, thatit, the polymer becomes gray and transparency is deteriorated bydeposition.

The content of the cobalt compound preferably satisfies the followingexpression:

    0.41≦Co≦1.2                                  (1)'

wherein Co is the same as defined in the above expression (1).

When the total amount of the calcium compound and the magnesium compoundis more than 6.0 moles, the resulting preform or bottle is whitened bydeposited particles derived from the residual catalysts during molding,whereby the transparency of the bottle is impaired. On the other hand,when the total amount is less than 2.0 moles, disadvantageously, anester exchange reaction becomes insufficient and the subsequentpolymerization reaction becomes slow.

The molar ratio of the magnesium compound to the calcium compound is ina range of 1.3 to 6.0. If the molar ratio is more than 6.0 or less than1.3, the deposition of particles derived from the residual catalystsoccurs, the resulting preform or bottle is whitened when it is molded,and the transparency of the bottle is impaired.

Further, to deactivate the ester exchange catalyst, it is preferablethat the amount of the phosphorus compound to be added as a stabilizeris in a molar ratio of 1.0 to 1.5, based on the total of the cobaltcompound, calcium compound and magnesium compound. When the molar ratiois less than 1.0, the ester exchange catalyst is not deactivatedcompletely and the thermal stability of the polymer is poor, whereby thepolymer is colored or its physical properties in the moldingdeteriorate. On the other hand, when the molar ratio is more than 1.5,the thermal stability of the polymer also lowers.

The cobalt compound, calcium compound and magnesium compound used in thepresent invention can be used, for example, as oxides, chlorides,carbonates or carboxylates. Out of these, acetates, i.e., cobaltacetate, calcium acetate and magnesium acetate are preferred.

Specific examples of the phosphorus compound include organic phosphoricacids and inorganic phosphoric acids. Organic phosphoric acids arephosphoric acid esters such as trimethyl phosphate, triethylenephosphate or tri-n-butyl phosphate. Inorganic phosphoric acids arehypophosphorous acid, phosphorous acid or orthophosphoric acid.

In the present invention, the germanium compound is used as apolymerization catalyst. From a viewpoint of hue, germanium dioxide ispreferably used as the germanium compound. Particularly, so-calledamorphous germanium dioxide having no crystal form is preferred. Whenthis amorphous germanium dioxide is used, the amount of depositedparticles in the polymer is smaller as compared with the case where anordinary crystalline germanium dioxide is used. Therefore, a polymerhaving a higher transparency can be obtained.

The amount of the germanium compound added is preferably in the range of1.0 to 3.0 moles. When the amount is smaller than 1.0 mole,polymerization reactivity lowers with the result of low productivity,while when the amount is larger than 3.0 moles, a polymer having poorthermal stability is obtained with the result that physical propertiesand hue are liable to deteriorate at the time of molding.

As for the time for adding the above catalysts, all of the cobaltcompound, calcium compound and magnesium compound are preferably addedduring from the start of the ester exchange reaction up to the initialstage thereof.

Meanwhile, the phosphorus compound can be added after the ester exchangereaction is substantially completed and before intrinsic viscosityreaches 0.3. The germanium compound is preferably added 10 minutes ormore before the phosphorus compound is added and before intrinsicviscosity reaches 0.2.

The polyethylene-2,6-naphthalene dicarboxylate of the present inventionhas an intrinsic viscosity of 0.55 to 0.75 dl/g as described above. Theintrinsic viscosity is preferably increased to a desired value bysolid-state polymerization in accordance with a commonly used methodafter melt polymerization. In this case, the intrinsic viscosity η! of aprepolymer obtained by the melt polymerization preferably satisfies thefollowing expression.

    0.40≦ η!≦0.60

When the intrinsic viscosity is smaller than 0.40 dl/g, the prepolymeris liable to crack when it is fabricated into chips, while when theintrinsic viscosity is larger than 0.60 dl/g, polymerization takes along time with the result of degradation in hue.

The polyethylene-2,6-naphthalene dicarboxylate of the present inventionobtained by the solid-state polymerization of the prepolymer asdescribed above preferably has an acetaldehyde content of 5 ppm or lessand a b value of +0.6 to -6.0

The polyethylene-2,5-naphthalene dicarboxylate of the present inventioncan be molded into a bottle by any known molding methods such asinjection blow molding, biaxial orientation stretch-blow molding,extrusion blow molding and the like.

As a typical example of the biaxial orientation stretch-blow molding, apolymer is first formed into a hollow cylindrical and bottomed preformhaving an appropriate shape (amorphous molded product) (having a lengthof 15 cm, an inner diameter of 2 cm and an outer diameter of 2.5 cm, forexample). This preform is preheated at a temperature higher than theglass transition point (Tg) of the polymer and the surface area of abody portion of the container is stretched to about 1.5 to 16 times byblowing. In this case, the heat treatment of the container can beappropriately carried out according to circumstances.

The amount of acetaldehyde contained in the obtained preform or bottleis preferably 40 ppm or less. When the amount is more than 40 ppm,disadvantageously, the flavor of the contents of the bottle is worsenedby an odor ascribed to acetaldehyde eluted into the contents of thebottle.

The transparency of the obtained bottle is preferably 5% or less interms of the bottle haze. When the haze is more than 5%, thetransparency of the bottle is poor, which is not preferred from aviewpoint of outer appearance.

As described above, it is understood that a preform for a bottle and abottle, which are molded of the polyethylene-2,6-naphthalenedicarboxylate of the present invention, are also objects the presentinvention is directed to.

Since the bottle of the present invention is excellent is transparencyand outer appearance, it can be used for various applications. Forexample, it can be used as a container for seasonings, oil, liquors,cosmetics, detergents, refreshing beverages, carbonated beverages,juice, water and beer. It is particularly useful as a container (bottle)to be recycled.

The following examples are given to further illustrate the presentinvention. "Parts" in the examples means "parts by weight". Physicalproperties were measured in accordance with the following methods.

(1) intrinsic viscosity η! (IV):

This was calculated from a measurement value at 35° C. using a mixedsolvent of tetrachloroethane and phenol (4:6).

(2) Col-L, a, b (hue):

The polymer was heated at 160° C. for 90 minutes in a dryer to becrystallized and then measured by a color machine (model CM-7500manufactured by Color Machine Co., Ltd.).

(3) haze:

The polymer was dried at 160° C. for 5 hours and molded into a preformhaving a weight of 63 g at a molding temperature of 305° C. at a moldingcycle of 40 seconds using an injection molding machine (100DMmanufactured by Meiki Seisakusho Co., Ltd.). The preform was formed intoa bottle having an inner volume of 1.0 liter and a body thickness of 400μm by a blow molding. The haze of the body portion of this bottle wasmeasured by a turbidimeter of Nippon Denshoku Kogyo Co., Ltd.

(4) amount of acetaldehyde:

The sample was frozen, ground and measured by the HS-GC (head space gaschromatography) of Hitachi Ltd. As for the amount of acetaldehyde of thebottle, a mouth portion of the bottle prepared in (3) was measured asthe sample.

(5) number of terminal carboxyl groups

The sample was dissolved in benzyl alcohol and titrated using causticsoda and phenol red as an indicator.

(6) impact resistance test

The sample bottle prepared in (3) was used. The bottle was filled with 4vol. of carbonated water and dropped onto a concrete floor from a heightof 1 m. The number of broken bottles out of 10 bottles is counted.

(7) alkali resistance test

The sample bottle prepared in (3) was used. After treated with a 3% NaOHaqueous solution at 85° C. for 6 hours, the sample bottle was measuredfor a change in the weight, and an impact resistance test described inthe above (6) was carried out.

EXAMPLE 1

100 Parts of dimethyl 2,6-naphthalenedicarboxylate and 51 parts ofethylene glycol (abbreviated as EG) were subjected to an ester exchangereaction in accordance with a commonly used method, using 0.005 part(0.02 mmol) of cobalt acetate tetrahydrate, 0.014 part (0.08 mmol) ofcalcium acetate monohydrate and 0.044 part (0.2 mmol) of magnesiumacetate tetrahydrate as ester exchange catalysts. 1.6 Parts (0.16 mmol)of a 1% EG solution of amorphous germanium dioxide was added, and then0.048 part (0.34 mmol) of trimethyl phosphate was added to complete theester exchange reaction.

Thereafter, a polycondensation reaction was carried out under conditionsof a high temperature and a high vacuum pressure in accordance with acommonly used method, and then the reaction product was formed intostrand-like chips. The intrinsic viscosity of the obtained prepolymerwas 0.5 dl/g and the polymerization time was 70 minutes. This prepolymerwas further polymerized in a solid state in accordance with a commonlyused method. The obtained polymer had an intrinsic viscosity of 0.65dl/g and contained acetaldehyde in an amount of 4 ppm, terminal carboxylgroups in an amount of 25 eq/ton and diethylene glycol in an amount of1.4 wt %.

EXAMPLES 2 TO 4 AND COMPARATIVE EXAMPLES 1 to 3

Polymers were obtained in the same manner as in Example 1 except thatthe amounts and ratio of cobalt acetate tetrahydrate, magnesium acetatetetrahydrate, calcium acetate monohydrate, trimethyl phosphate andgermanium dioxide were changed as shown in Table 1. The qualities andevaluation results of the obtained polymers are also shown in Tables 1and 2.

                                      TABLE 1                                     __________________________________________________________________________                                               Characteristic                                             Characteristic properties of                                                                     properties of                      Amount of catalyst                                                                          Prepolymer                                                                              solid-state polymer                                                                              bottle                             (mol/ton)     Polymerization                                                                        η!                                                                           η!                                                                          Col-                                                                              AA  DEG COOH                                                                              Haze                                                                             AA                              Co    Ca                                                                              Mg                                                                              P Ge                                                                              time (minutes)                                                                       (dl/g)                                                                           (dl/g)                                                                           L/b (ppm)                                                                             (wt %)                                                                            (eq/T)                                                                            (%)                                                                              (ppm)                           __________________________________________________________________________    Ex. 1                                                                             0.2                                                                             0.8                                                                             2.0                                                                             3.4                                                                             1.6                                                                             70     0.50                                                                             0.65                                                                             83/-1.0                                                                           4.0 1.4 25  1.0                                                                              20                              Ex. 2                                                                             0.2                                                                             0.8                                                                             2.1                                                                             3.4                                                                             1.6                                                                             60     0.45                                                                             0.64                                                                             83/-1.5                                                                           2.4 1.5 20  1.1                                                                              18                              Ex. 3                                                                             0.6                                                                             0.8                                                                             2.1                                                                             4.9                                                                             2.5                                                                             71     0.50                                                                             0.71                                                                             80/-1.8                                                                           2.4 2.0 19  1.2                                                                              35                              Ex. 4                                                                             1.0                                                                             0.8                                                                             2.1                                                                             4.3                                                                             1.3                                                                             75     0.46                                                                             0.70                                                                             75/-4.0                                                                           1.8 1.8 17  1.1                                                                              30                              Comp.                                                                             0.6                                                                             0.8                                                                             2.1                                                                             3.9                                                                             0.9                                                                             130    0.62                                                                             0.76                                                                             70/5.0                                                                            9.6 0.7 37  5.6                                                                              50                              Ex. 1                                                                         Comp.                                                                             0.1                                                                             0.8                                                                             2.1                                                                             3.4                                                                             1.6                                                                             72     0.51                                                                             0.54                                                                             84/0                                                                              40  0.6 38  6.0                                                                              57                              Ex. 2                                                                         Comp.                                                                             0.1                                                                             0.8                                                                             2.1                                                                             3.4                                                                             1.6                                                                             71     0.50                                                                             0.54                                                                             83/0.5                                                                            35  3.5 37  6.3                                                                              60                              Ex. 3                                                                         __________________________________________________________________________     Ex.: Example, Comp. Ex.: Comparative Example                                  In Tables 1 and 3, AA indicates the content of acetaldehyde, COOH the         number of terminal carboxyl groups and DEG the content of diethylene          glycol                                                                   

                  TABLE 2                                                         ______________________________________                                               Impact      Alkali resistance                                                 resistance  Weight reduc-                                                                             Impact                                                (out of 10 bottles)                                                                       tion (%)    resistance                                     ______________________________________                                        Example 1                                                                              no broken     0.2         no broken                                           bottles                   bottles                                    Comparative                                                                            4 broken bottles                                                                            1.0         7 broken                                   Example 1                          bottles                                    Comparative                                                                            8 broken bottles                                                                            1.2         10 broken                                  Example 2                          bottles                                    ______________________________________                                    

EXAMPLE 5

100 Parts of dimethyl 2,6-naphthalenedicarboxylate and 51 parts ofethylene glycol (abbreviated as EG) were subjected to an ester exchangereaction in accordance with a commonly used method using 0.02 part (0.8mole) of cobalt acetate tetrahydrate, 0.014 part (0.8 mole) of calciumacetate monohydrate and 0.044 part (2.1 moles) of magnesium acetatetetrahydrate as ester exchange catalysts. 1.58 parts (1.6 moles) of a 1%EG solution of amorphous germanium dioxide was added, and then 0.047part (3.4 moles) of trimethyl phosphate was added to complete the esterexchange reaction. Thereafter, a polycondensation reaction was carriedout under the conditions of a high temperature and at a high vacuumpressure in accordance with a commonly used method, and the reactionproduct was formed into strand-like chips. The intrinsic viscosity ofthe obtained prepolymer was 0.5 dl/g and the polymerization time was 70minutes. This prepolymer was further polymerized in a solid state inaccordance with a commonly used method. The obtained polymer had anintrinsic viscosity of 0.7 dl/g and an acetaldehyde content of 2 ppm.

EXAMPLES 6 to 9

Polymers were obtained in the same manner as in Example 5 except thatthe amounts and ratio of cobalt acetate tetrahydrate, magnesium acetatetetrahydrate, calcium acetate monohydrate, trimethyl phosphate andgermanium dioxide were changed as shown in Table 3. The qualities andevaluation results of the obtained polymers are shown in Table 3.

                                      TABLE 3                                     __________________________________________________________________________                                                           Characteristic                                 Prepolymer                                                                              Characteristic properties                                                                          properties             Amount of catalyst      Polymeri- solid-state polymer  of bottle              (mol)        Ca +                                                                             Mg/                                                                              P/(Co +                                                                            zation time                                                                          η!                                                                            η!                                                                          Col  AA  DEG  COOH                                                                              Haze                                                                              AA                 Co   Ca                                                                              Mg                                                                              P Ge                                                                              Mg Ca Ca + Mg)                                                                           (minutes)                                                                           (dl/g)                                                                            (dl/g)                                                                           L/b  (ppm)                                                                             (wt %)                                                                             (eq/T)                                                                            (%) (ppm)              __________________________________________________________________________    Ex. 5                                                                            0.6                                                                             0.8                                                                             2.1                                                                             3.8                                                                             1.6                                                                             2.9                                                                              2.6                                                                              1.1  70    0.50                                                                              0.70                                                                             81/-2.0                                                                            2.0 0.9  25  1.0 34                 Ex. 6                                                                            0.6                                                                             1.2                                                                             1.7                                                                             3.8                                                                             1.6                                                                             2.9                                                                              1.4                                                                              1.1  73    0.50                                                                              0.70                                                                             81/-2.1                                                                            2.4 1.4  24  1.1 33                 Ex. 7                                                                            0.6                                                                             0.8                                                                             2.1                                                                             4.9                                                                             2.5                                                                             2.9                                                                              2.6                                                                              1.4  60    0.50                                                                              0.70                                                                             80/-1.8                                                                            2.3 1.8  26  1.2 34                 Ex. 8                                                                            1.0                                                                             0.8                                                                             2.1                                                                             4.3                                                                             1.3                                                                             2.9                                                                              2.6                                                                              1.1  75    0.50                                                                              0.70                                                                             75/-4.0                                                                            2.5 2.0  24  1.1 34                 Ex. 9                                                                            0.6                                                                             0.8                                                                             2.1                                                                             3.8                                                                             1.6                                                                             2.9                                                                              2.6                                                                              1.1  70    0.50                                                                              0.74                                                                             80/-1.7                                                                            3.0 2.5  25  1.0 35                 __________________________________________________________________________     Ex.: Example                                                                  Comp. Ex.: Comparative Example                                           

The polyethylene-2,6-naphthalene dicarboxylate of the present inventionis excellent in hue, thermal stability and transparency, and provides abottle product of high commercial product value, which is hardlywhitened at the time of molding. Further, a bottle molded of thispolyethylene-2,6-naphthalene dicarboxylate has properties suitable foruse as a bottle which can be recycled.

What is claimed is:
 1. A polyethylene-2,6-naphthalene dicarboxylateresin for molding a bottle, which comprises 2,6-naphthalenedicarboxylicacid as a main acid component and ethylene glycol as a main glycolcomponent and contains a germanium compound as a polycondensationcatalyst, and has an intrinsic viscosity of 0.55 to 0.75 dl/g, aterminal carboxyl group content of 32 eq/ton or less, an acetaldehydecontent of 8 ppm or less and a diethylene glycol component content of0.8 to 3.0 wt %.
 2. The polyethylene-2,6-naphthalene dicarboxylate resinof claim 1, which has an intrinsic viscosity of 0.58 to 0.73 dl/g. 3.The polyethylene-2,6-naphthalene dicarboxylate resin of claim 1, whichhas a terminal carboxyl group content of 30 eq/ton or less.
 4. Thepolyethylene-2,6-naphthalene dicarboxylate resin of claim 1, which hasan acetaldehyde content of 6 ppm or less.
 5. Thepolyethylene-2,6-naphthalene dicarboxylate resin of claim 1, which has adiethylene glycol component content of 1.0 to 2.8 wt %.
 6. Thepolyethylene-2,6-naphthalene dicarboxylate resin of claim 1, which has ab value in the range of +0.6 to -6.0.
 7. Thepolyethylene-2,6-naphthalene dicarboxylate resin of claim 1, whichfurther contains a cobalt compound, a magnesium compound, a calciumcompound and a phosphorus compound in amounts that satisfy the followingexpressions:

    0.12 ≦Co≦1.2                                 (1)

    2.0≦Mg+Ca≦6.0                                (2)

    1.3≦Mg/Ca≦6.0                                (3)

    1.0≦P/(Co+Mg+Ca)≦1.5                         (4)

    1.0≦Ge≦3.0                                   (5)

wherein Co, Mg, Ca, P and Ge indicate the number of moles of cobaltatoms in the cobalt compound, the number of moles of magnesium atoms inthe magnesium compound, the number of moles of calcium atoms in thecalcium compound, the number of moles of phosphorus atoms in thephosphorus compound and the number of moles of germanium atoms in thegermanium compound, based on 1 ton of acid components constituting thepolymer, respectively.
 8. The polyethylene-2,6-naphthalene dicarboxylateresin of claim 7, wherein the amount of the cobalt compound satisfiesthe following expression:

    0.41≦Co≦1.2                                  (1)'

wherein Co is the same as defined in the above expression (1).
 9. Thepolyethylene-2,6-naphthalene dicarboxylate resin of claim 8, which hasan acetaldehyde content of 5 ppm or less.
 10. Thepolyethylene-2,6-naphthalene dicarboxylate resin of claim 7, which has ab value in the range of +0.6 to -6.0.
 11. A preform for a bottle, whichis formed from the polyethylene-2,6-naphthalene dicarboxylate resin ofclaim
 1. 12. A bottle formed from the polyethylene-2,6-naphthalenedicarboxylate resin of claim 1.